docs: update Windows installation documentation for Windows 7/Server 2008 R2 and newer

Update to the Windows installation documentation following my setup experience
on our production server.

Changes :

* Use of Microsoft Visual C++ Compiler for Python 2.7 which simplifies matter
versus installing Visual Studio 2008 Express, because it removes the need
to use the Visual Studio 2008 command prompt. It is also a lot smaller to
download and install. Unfortunately, this means the instruction will only
work on the same platforms as those which are supported by the compiler.
* For that reason, I split the documentation into newer and older Windows.
* Added more explanations where I feel it was necessary based on my experience
* Added explanation on Git (Warning : I did not try this part)
* Instructions assumes x64 instead of Win32.
* Clarified titles
* Grammar

Potential issues:

* I have the user install pip system wide so that virtualenv installation is
easier (especially if using Python 2.7.9 which already includes pip). One
may prefer to install virtualenv and the pip in the virtual environment (I
know of no good reason, but it could happen).
* Removed some line feeds that I found useless. I do not know the .rst
format, they might be needed. It makes no difference when the documentation
is generated using make.bat, so I am not sure

Potential improvements:

* Instructions on using srvany.exe to install as a Windows service
* Instructions to make a reverse proxy using Apache
* Instructions to make a reverse proxy using IIS